Optical configurations for optical field mappings for back-scanned and line-scanned imagers

Abstract

Examples are directed to optimal field mappings that provide the highest contrast images for back-scanned imaging methods. The mappings can be implemented for back-scanned imaging with afocal optics including an anamorphic field correcting assembly configured to implement a non-rotationally symmetric field mapping between object space and image space to adjust distortion characteristics of the afocal optics to control image wander on a focal plane array. The anamorphic field correcting assembly can include one or more mirrors or lenses having non-rotationally symmetric aspherical departures.

What is claimed is: 
     
       1. An imaging method comprising:
 receiving electromagnetic radiation from a viewed scene with afocal optics; 
 directing the electromagnetic radiation from the afocal optics to an imager; 
 focusing the electromagnetic radiation with the imager onto an imaging sensor to form an image, the image being centered about an optical axis of the afocal optics that passes through an exit pupil of the afocal optics to the imager; 
 back-scanning the electromagnetic radiation with a back-scan mirror optically coupled to the afocal optics and positioned proximate the exit pupil of the afocal optics to stabilize the image on the imaging sensor; and 
 adjusting distortion characteristics of the afocal optics with a field correcting assembly to control image wander for off-axis image points during the back-scanning operation, the field correcting assembly including at least one anamorphic field correction optical element configured to implement a non-rotationally symmetric field mapping between image space and object space to set the distortion characteristics of the afocal optics. 
 
     
     
       2. The imaging method of  claim 1  wherein the non-rotationally symmetric field mapping is defined by θ i =A mag θ 0  and ϕ i =A mag ϕ 0 , wherein θ i  and ϕ i  are ray angles in image space, θ o  and ϕ o  are ray angles in object space, and A mag  is a magnification of the afocal optics. 
     
     
       3. The imaging method of  claim 1  wherein receiving the electromagnetic radiation with the afocal optics includes receiving the electromagnetic radiation through an entrance pupil of the afocal optics with an on-axis telescope and directing the electromagnetic radiation from the on-axis telescope along the optical axis to a pupil relay. 
     
     
       4. The imaging method of  claim 3  further comprising reimaging the electromagnetic radiation with the pupil relay onto the back-scan mirror via the exit pupil of the afocal optics. 
     
     
       5. The imaging method of  claim 4  further comprising scanning a field-of-view of the on-axis telescope over a field of regard using a head mirror. 
     
     
       6. The imaging method of  claim 5  further comprising rotating the image using a derotation element positioned between the on-axis telescope and the pupil relay. 
     
     
       7. The imaging method of  claim 1  wherein the at least one anamorphic field correcting optical element includes first and second lenses, and wherein the method further comprises forming an intermediate image between the first and second lenses. 
     
     
       8. The imaging method of  claim 1  wherein the non-rotationally symmetric field mapping is based on a polar coordinate system. 
     
     
       9. An imaging method for back-scanned imaging systems, the imaging method comprising:
 receiving electromagnetic radiation from a viewed scene with afocal optics, the afocal optics including a telescope, a pupil relay; 
 scanning a field-of-view of the afocal optics over a field-of-regarding using a head mirror; 
 directing the electromagnetic radiation from the afocal optics to an imager; 
 focusing the electromagnetic radiation with the imager onto an imaging sensor to form an image, the image being centered about an optical axis of the afocal optics that extends between an entrance pupil of the afocal optics and an exit pupil of the afocal optics; 
 back-scanning the electromagnetic radiation with a back-scan mirror optically coupled to the afocal optics and positioned proximate the exit pupil of the afocal optics between the afocal optics and the imager to stabilize the image on the imaging sensor; and 
 adjusting distortion characteristics of the afocal optics with a field correcting assembly to control image wander for off-axis image points during the back-scanning operation, the field correcting assembly including at least one anamorphic field correction optical element configured to implement a non-rotationally symmetric field mapping between image space and object space to set the distortion characteristics of the afocal optics, and the field correcting assembly being implemented in at least one of the telescope and the pupil relay. 
 
     
     
       10. The imaging method of  claim 9  wherein the non-rotationally symmetric field mapping is defined by θ i =A mag θ 0  and ϕ i =A mag ϕ 0 , wherein θ i  and ϕ i  are ray angles in image space, θ o  and ϕ o  are ray angles in object space, and A mag  is a magnification of the afocal optics. 
     
     
       11. The imaging method of  claim 9  wherein the afocal optics further includes a derotation element positioned between the telescope and the pupil relay, the method further comprising compensating for image rotation introduced by the head mirror with the derotation element. 
     
     
       12. The imaging method of  claim 11  wherein field correcting assembly is implemented in the pupil relay, and wherein receiving the electromagnetic radiation with the afocal optics and directing the electromagnetic radiation to the imager include:
 receiving the electromagnetic through an entrance pupil of the afocal optics with the telescope, the telescope being an afocal telescope; 
 directing the electromagnetic radiation from the telescope to the pupil relay via the derotation element; and 
 reimaging the electromagnetic radiation with the pupil relay onto the back-scan mirror via the exit pupil of the afocal optics. 
 
     
     
       13. The imaging method of  claim 12  wherein the field correcting assembly including a pair of lenses, the method further comprising forming an intermediate image between first and second lenses of the pair of lenses. 
     
     
       14. The imaging method of  claim 9  wherein the field correcting assembly includes a deformable mirror, and wherein adjusting distortion characteristics of the afocal optics includes adjusting a shape of the deformable mirror to alter the distortion characteristics of the afocal telescope and control the image wander for off-axis field points during the back-scan operation.